Gnosticism (from Ancient Greek γνωστικός (gnōstikós) 'having knowledge'; Koine Greek: [ɣnostiˈkos]) is a collection of religious and philosophical ideas and systems that coalesced in the late first century AD among early Christian sects and the sects of other religions. These diverse groups emphasized personal spiritual knowledge (gnosis) above the authority, traditions, and proto-orthodox teachings of organized religious institutions. The Gnostic worldview generally distinguished between a hidden, uncorrupted supreme being and a flawed demiurge responsible for creating material reality. Gnostics held this material existence to be evil and believed the principal element of salvation to be direct knowledge of the supreme divinity, attained via mystical or esoteric insight. Many Gnostic texts deal not in concepts of sin and repentance, but with illusion and enlightenment.

Although the exact origins of Gnosticism cannot be traced, Gnostic writings flourished among certain Christian groups in the Mediterranean world around the second century, when the Early Church Fathers denounced them as heresy. Efforts to destroy these texts were largely successful, resulting in the survival of very little writing by Gnostic theologians. Nonetheless, early Gnostic teachers such as Valentinus saw themselves as Christians. In the Gnostic Christian tradition, Christ was seen as a divine being, which had taken human form in order to lead humanity back to recognition of its own divine nature.

Judean–Israelite Gnosticism, including the Mandaeans and Elkesaites, blended Jewish-Christian ideas with Gnostic beliefs focused on baptism and the cosmic struggle between light and darkness, with the Mandaeans still practicing ritual purity today. Syriac–Egyptian groups like Sethianism and Valentinianism combined Platonic philosophy and Christian themes, seeing the material world as flawed but not wholly evil. Other traditions include the Basilideans, Marcionites, Thomasines, and Manichaeism, known for its cosmic dualism. After declining in the Mediterranean, Gnosticism persisted near the Byzantine Empire and resurfaced in medieval Europe with groups like the Paulicians, Bogomils, and Cathars, who were accused of Gnostic traits. Islamic and medieval Kabbalistic thought also reflect some Gnostic ideas, while modern revivals and discoveries such as the Nag Hammadi texts have influenced numerous thinkers and churches up to the present day.

For centuries, most scholarly knowledge about Gnosticism was limited to the anti-heretical writings of early Christian figures such as Irenaeus of Lyons and Hippolytus of Rome. There was a renewed interest in Gnosticism after the 1945 discovery knowledge of Egypt’s Nag Hammadi library, a collection of rare early Christian and Gnostic texts, Gnosticism came mainly from biased and incomplete heresiological writings; the recovered Gnostic texts, including the Gospel of Thomas and the Apocryphon of John; the recovered Gnostic texts revealed a very diverse and complex early Christian landscape. Some scholars say Gnosticism may contain historical information about Jesus from the Gnostic viewpoint, although the majority conclude that apocryphal sources, Gnostic or not, are later than the canonical sources and many, such as the Gospel of Thomas, depended on or used the Synoptic Gospels. Elaine Pagels has noted the influence of sources from Hellenistic Judaism, Zoroastrianism, and Middle Platonism on the Nag Hammadi texts. Since the 1990s, scholars have debated whether “Gnosticism” is a form of early Christianity and if it is an artificial category created by early orthodox leaders to label heresies. Academic studies of Gnosticism have evolved from viewing it as a Christian heresy or Greek-influenced aberration to recognizing it as a diverse set of movements with complex Jewish, Persian, and philosophical roots; consequently, modern scholars question the usefulness of "Gnosticism" as a unified category and favor more precise classifications based on texts, traditions, and socio-religious contexts.

Gnosis is a feminine Greek noun which means "knowledge" or "awareness". It and the associated verb are often used for personal knowledge, as compared with intellectual knowledge (Greek verb εἴδειν eídein). A related term is the adjective gnostikos, "of or for knowledge", a reasonably common adjective in Classical Greek.

By the Hellenistic period, it began also to be associated with Greco-Roman mysteries, becoming synonymous with the Greek term mysterion. Consequentially, Gnosis often refers to knowledge based on personal experience or perception.^{[citation needed]} In a religious context, gnosis is mystical or esoteric knowledge based on direct participation with the divine. In most Gnostic systems, the sufficient cause of salvation is this "knowledge of" ("acquaintance with") the divine. It is an inward "knowing", comparable to that encouraged by Plotinus (neoplatonism), and differs from proto-orthodox Christian views. Gnostics are "those who are oriented toward knowledge and understanding – or perception and learning – as a particular modality for living". The usual meaning of gnostikos in Classical Greek texts is "learned" or "intellectual", such as used by Plato in the comparison of "practical" (praktikos) and "intellectual" (gnostikos). Plato's use of "learned" is fairly typical of Classical texts.

Sometimes employed in the Septuagint translation of the Hebrew Bible, the adjective is not used in the New Testament, but Clement of Alexandria who speaks of the "learned" (gnostikos) Christian quite often, uses it in complimentary terms. The use of gnostikos in relation to heresy originates with interpreters of Irenaeus. Some scholars consider that Irenaeus sometimes uses gnostikos to simply mean "intellectual", whereas his mention of "the intellectual sect" is a specific designation. The term "Gnosticism" does not appear in ancient sources, and was first coined in the 17th century by Henry More in a commentary on the seven letters of the Book of Revelation, where More used the term "Gnosticisme" to describe the heresy in Thyatira. The term Gnosticism was derived from the use of the Greek adjective gnostikos (Greek γνωστικός, "learned", "intellectual") by St. Irenaeus (c. 185 AD) to describe the school of Valentinus as he legomene gnostike haeresis "the heresy called Learned (gnostic)".

The origins of Gnosticism are obscure and still disputed. Alexandria was of central importance for the birth of Gnosticism. Gnosticism is strongly influenced by Middle Platonism and its theory of forms. Elaine Pagels has noted the influence of sources from Hellenistic Judaism, Zoroastrianism, and Middle Platonism on the Nag Hammadi texts. The Christian ecclesia (i. e. congregation, church) was of Jewish–Christian origin, but also attracted Greek members, and various strands of thought were available, such as "Judaic apocalypticism, speculation on divine wisdom, Greek philosophy, and Hellenistic mystery religions." The proto-orthodox Christian groups called Gnostics a heresy of Christianity.